As a light source, projectors often use a high-pressure discharge lamp, which provides nearly point-source light. In particular, use of high-pressure mercury lamps is common. A high-pressure mercury lamp includes an arc tube containing mercury as a light-emitting material, as well as a pair of tungsten electrodes disposed substantially opposite each other. The high-pressure mercury lamp emits light by producing arc discharge between the electrodes.
Widely-used, conventional lighting devices for such a high-pressure mercury lamp light the lamp by providing a constant current Ic until a predetermined time passes after the start of lighting, for example until reaching a rated lamp wattage Pr. After the predetermined time passes, the lamp power is constant at the rated lamp wattage Pr. During constant power supply, if the lamp voltage drops too low, the lamp current may become too large, damaging the electronic components in the device. Therefore, the lamp current is limited to be equal to or less than a predetermined value (hereinafter referred to as the current limit Ia). Furthermore, the current Ic is normally set to the value of the current limit Ia.
In the above-described conventional lighting device for a high-pressure mercury lamp, when the constant current supply at the current Ic switches to constant power supply at the rated lamp wattage Pr, the provided power and current are both large (rated lamp wattage Pr, current Ic). In particular, it is known that at this time, the temperature of the pair of electrodes rises notably, causing damage to the electrodes.
To reduce such damage to the electrodes, one proposed lighting device for a high-pressure mercury lamp provides a constant current Id that is smaller than the conventional current Ic from the start of lighting until a predetermined time has passed, thus reducing the current as compared to a conventional configuration when switching to constant power supply at the rated lamp wattage Pr (see Patent Literature 1).
In the lighting device of Patent Literature 1, the current Id is set using the rated lamp wattage Pr and the upper limit Vhigh of the tolerance in the rated lamp voltage that is specified taking into account variation upon manufacturing of the high-pressure mercury lamp (Id<Pr/Vhigh). On the other hand, the current Ic in the above conventional lighting device is the same size as the current limit Ia. The current limit Ia is generally set using the rated lamp wattage Pr and the lower limit Vlow, of the tolerance (Ia=Pr/Vlow). Accordingly, the current Id becomes smaller than the current limit Ia and the current Ic. Since the current decreases, the increase in temperature of the pair of electrodes is correspondingly moderated as compared to the above conventional lighting device, which is considered to reduce damage to the electrodes.
In the graphs in FIGS. 14A-14C, the lamp power, lamp voltage, and lamp current over time when a lamp is lit by the lighting device recited in Patent Literature 1 are shown as solid lines. The relationships when a lamp is lit by the above conventional lighting device are shown as lines with alternate long and two short dashes. The rated lamp wattage of the high-pressure mercury lamp in these graphs is 250 W, the rated lamp voltage is 80 V, and the tolerance of the rated lamp voltage is 62.5 V to 95 V.
As shown in FIGS. 14A-14C, with the lighting device recited in Patent Literature 1, a current Id (2.5 A) that is lower than the current Ic (4 A) in the above conventional lighting device is provided from the start of lighting until the lamp voltage reaches 80 V. The current when switching to the constant power supply at the rated lamp wattage of 250 W is 3.125 A, a smaller value than the current Ic.